Indonesian throughflow controlled the westward extent of the Indo-Pacific Warm Pool during glacial-interglacial intervals

Global and Planetary Change 183 (2019) 103031

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Indonesian throughflow controlled the westward extent of the Indo-Pacific Warm Pool during glacial-interglacial intervals

T

Rajeev Saraswata, , D.P. Singha, David W. Leab, A. Mackensenc, D.K. Naikd ⁎

a

National Institute of Oceanography, Dona Paula, Goa, India Alfred Wegner Institute for Polar and Marine Research, Bremerhaven, Germany c Department of Earth Science and Marine Science Institute, University of California, Santa Barbara 93106-9630, USA d Banaras Hindu University, Varanasi, India b

ARTICLE INFO

ABSTRACT

Keywords: Indian Ocean Indo-Pacific Warm Pool Mg/Ca δ18O Glacial-interglacial

The tropical atmospheric circulation is strongly influenced by the sea surface temperature (SST) of the warmest region of the world ocean, the Indo-Pacific Warm Pool (IPWP). The effect of recent anomalous Indian Ocean warming on the extent of the IPWP remains uncertain. Past SST records from the tropical Indian Ocean, associated with different temporal boundary conditions, can help shed light on how climate change impacts the IPWP and associated monsoonal processes. Here, we reconstruct the seawater temperature and precipitation changes (from Mg/Ca and δ18O of surface-dwelling planktic foraminifera Globigerinoides ruber) of the last ~184 kyr BP from the westernmost margin of the IPWP. The data is then used to reconstruct the spatial extent and intensity of the IPWP structure as well as its relationship with other regional phenomena. A consistently warmer (> 28 °C) central equatorial Pacific Ocean and significantly cooler equatorial western Pacific and the Indian Ocean suggests that the warm pool extent reduced considerably during MIS6 as well as the last glacial interval, leading to a strong La-Nina like condition. The most enriched seawater δ18O (δ18Osw) during MIS6, was in the westernmost IPWP, suggesting weakened precipitation, despite La-Lina like conditions. The westernmost part of the IPWP became warmer than the threshold 28 °C during MIS5e and the early Holocene, suggesting the westward extension of the IPWP. The eastern equatorial Pacific Ocean also warmed considerably during MIS5e, suggesting a weaker Walker Circulation and a stronger El-Nino. The difference in δ18Osw decreased considerably during MIS5e and is attributed to increased precipitation/freshwater runoff in the central equatorial Indian Ocean. The increase in δ18Osw (~1.5‰) between ~55 kyr until the last glacial maximum, in a majority of the Indian Ocean records, suggests progressively weakened precipitation. A majority of the records have a uniform SST anomaly throughout the Holocene. A warmer equatorial Indian Ocean during interglacials suggests a strong modulation of the IPWP extent by the transport of warm water through the Indonesian throughflow region. The large drop in δ18Osw despite the comparable change in SST during the last glacial interval suggests that monsoon forcing has been much stronger in altering the surface hydrography in the Indian Ocean than the central Pacific Ocean.

1. Introduction The Indo-Pacific warm pool (IPWP) is the world's largest heat reservoir with a surface area > 30 × 106 km2 (De Deckker, 2016). The SST in the IPWP remains warmer than 28 °C throughout the year (Wyrtki, 1989) and the region modulates large-scale deep convection (Graham and Barnett, 1987). Earlier, the oceanic area between 10°N to 10°S with its center at 170°E was suggested as the IPWP (Wyrtki, 1989). Later, however, it was realized that the 28 °C isotherm extends up to 25°N to 25°S (Weller et al., 2016). The IPWP extends through the Indian

⁎

and the Pacific Ocean. It is the largest source of heat and moisture and drives the majority of the atmospheric circulations including Walker and Hadley circulation. The recent hiatus in global warming is also attributed to the excess heat storage in both the Pacific and the Indian Ocean (Lee et al., 2015; Arora et al., 2016). The mean state of the IPWP is crucial for the precipitation in entire Asia, the most populated region of the world (De Deckker, 2016). The ascending limb of the Hadley circulation, known as Intertropical Convergence Zone (ITCZ) is associated with the area of maximum heat on the planet and influences the hydrography of the entire tropical region.

Corresponding author. E-mail address: [email protected] (R. Saraswat).

https://doi.org/10.1016/j.gloplacha.2019.103031 Received 12 December 2018; Received in revised form 13 August 2019; Accepted 10 September 2019 Available online 11 September 2019 0921-8181/ © 2019 Elsevier B.V. All rights reserved.

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The warm and intense low-pressure zone is also essential to maintain the zonal Walker circulation and meridional Hadley circulation. At present, the eastern part of the IPWP is relatively warmer than the western part. The preferential heat storage in either of these oceans will alter the zonal temperature gradient. The Pacific and the Indian Ocean part of the IPWP are connected through the Indonesian islands. Nearly 15 Sv (Sv = 106 m3 s−1) of low saline warm water passes into the Indian Ocean through the Indonesian Throughflow (ITF) (Castruccio et al., 2013). ITF is a vital route for the transfer of heat and water from the equatorial western Pacific Ocean to the eastern Indian Ocean and thus to maintain the IPWP extent and intensity in the western Pacific and the eastern Indian Ocean (Lee et al., 2002). The shallow sills in ITF are critical for this cross-basin exchange of water and are prone to sealevel variations. The reduced cross-basing exchange due to lower glacial sea level and exposed Sunda shelf weakened the Walker circulation. The weakened Walker circulation was responsible for drier conditions throughout the IPWP and wetter condition in western Indian and eastern Pacific Ocean during the last glacial maximum (LGM) (Dinezio and Tierney, 2013; DiNezio et al., 2018), thus implying a close link between the state of IPWP and precipitation in the tropical regions (Kumar et al., 2007). The extent and intensity of IPWP is, therefore, crucial for the rainfall in the tropics and thus has far-reaching implications for the vast human population dependent on rain. The increase in climate sensitivity of the equatorial Pacific SST within the IPWP, beyond a certain atmospheric CO2 level, has also been suggested to be a significant factor in the transition from glacial to interglacial (Lo et al., 2017). Therefore, a better understanding of the IPWP structure during different boundary conditions can help in assessing its response to the anthropogenic changes. Several attempts have been made to understand the structure, extent and intensity of IPWP during the past (Gagan et al., 2004; Linsley et al., 2010; Saraswat et al., 2007; Visser et al., 2003; Xu et al., 2010; Li et al., 2011; Qiu et al., 2014; Tachikawa et al., 2014; Regoli et al., 2015). A majority of the records have focused on the last glacial-interglacial transition. A 1.2 °C cooling of the eastern equatorial Pacific SST during the LGM was inferred from a core (Koutavas et al., 2002) whereas other records (ODP 1242 and ME0005A-43JC) suggest a similar cooling in both the eastern and western Pacific Ocean (Benway et al., 2006). The Indian Ocean was ~3–4 °C cooler during the LGM (Saraswat et al., 2005; Saraswat et al., 2013; Anand et al., 2008; Govil and Naidu, 2010). Although the western Pacific warm pool was continuously present during the LGM, the intensity was weak (Thunnel et al., 1994). The Hadley and Walker circulation weakened, ITCZ shifted to a more southerly position and El-Nino like condition prevailed during the LGM (Koutavas et al., 2002). The weakening of the

Pacific Walker circulation during the LGM was accompanied by the strengthening of the Indian Walker circulation (Niedermeyer et al., 2014; Mohtadi et al., 2017). The changes in tropical processes, including a southward shift in the ITCZ was not only synchronous with northern hemisphere cold events, but was also coupled with oceanic advection and mixing, and thus responsible for salinity increase in both the ITF region and the eastern tropical Pacific Ocean (Gibbons et al., 2014). A few long-term records suggest equally cooler (~2.8 °C) equatorial eastern and western Pacific Ocean during the LGM and further that the tropical cooling played a major role in driving ice-age climate (Lea et al., 2000; Russon et al., 2010). A comparison of the SST record of the last glacial and interglacial interval from the central equatorial Indian Ocean and equatorial western Pacific, however, suggests that the Indian Ocean was warmer than the Pacific Ocean during the last glacial interval (Saraswat et al., 2007). Inter-Hemispherical synchrony was persistent in the Indian Ocean (Bard et al., 1997) but asymmetry marked the Pacific Ocean (Russon et al., 2010). The change in monsoon intensity was likely responsible for the inter-hemispheric SST heterogeneity as it affects the annual mean SST and seasonality. The salinity in the Indian Ocean increased by 1.5 psu during the LGM (Mahesh et al., 2011). The short-term salinity records suggest a 1.5 psu change in salinity in the equatorial western Pacific Ocean during the Holocene (Stott et al., 2004) and 3–4 psu change in the equatorial eastern Pacific Ocean (Benway et al., 2006). The δ18Osw in northern hemisphere freshened, whereas δ18Osw south of the equator was not significantly different between the last glacial period and the Holocene. The negligible glacial-Holocene difference in δ18Osw south of the equator, suggests that additional factors affect annual mean SST in the IPWP (Mohtadi et al., 2010). The limited long-term SST and salinity records from the central equatorial Indian Ocean, the westernmost extent of the IPWP, hamper a proper assessment of the change in IPWP extent and intensity during the glacial-interglacial intervals. Additional past sea surface temperature records from the core of the eastern and western parts of the IPWP can provide further insight into the temporal stability of the IPWP. The temperature data, coupled with the precipitation-evaporation changes can further help in understanding the link between the Pacific Ocean temperature and Indian monsoon. 2. Materials and methods 2.1. Core location and oceanographic setting The gravity core SK237 GC09 (12°00.59′N, 70°52.20′E) was

Fig. 1. Map of annual average SSTs in the IPWP region and core locations (Locarnini et al., 2006). The spatial coverage of the modern Indo-Pacific Warm Pool is delineated by the 28 °C SST isotherm (black line). The blue rectangle is the core location of SK239 GC09 (lat 12°00.59′N, long 70°52.20′E, depth 3001 m). The black dots are the previously published core records from the IPWP region, discussed in the text. The details of the cores are available in Table 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2

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collected from 3001 m water depth during the 237th cruise of the ORV Sagar Kanya (Fig. 1). The core was collected from the southeastern Arabian Sea at the western extreme of the modern IPWP, well above the calcium carbonate compensation depth (Banakar et al., 1998). The southeastern Arabian Sea experiences extreme warming (> 30 °C) before the monsoon vortex in the form of the mini warm pool (Vinayachandran et al., 2007). The salinity in the southeastern Arabian Sea is mainly controlled by riverine input from the Western Ghats during the southwest monsoon (Chauhan et al., 2011) and the seasonally reversing coastal currents including the intrusion of low saline water (33.3 psu; Rao et al., 2011) from the Bay of Bengal during the northeast monsoon season (Shetye et al., 1996; Wijesekera et al., 2015). The mean annual temperature at the core site is 28.59 °C and salinity is 35.80 psu. The core was 430 cm long and sub-sampled at 1 cm interval. The top 300 cm section of the core was used.

discussed here covers the last 184 kyr. The average sedimentation rate is 1.7 cm/kyr (varying from 0.4 cm/kyr to 2.8 cm/kyr). The age at the core top is 3.1 kyr BP, suggesting a possible loss of the top few cm sediments while retrieving the core. Given that the bioturbation lock-in depth might be on the order of a few centimeters per kyr, the relatively low sedimentation rate is also likely to result in older age at the surface, due to bioturbation. The magnitude of glacial-interglacial changes could probably be impacted by bioturbation. However, it is difficult to assess the exact implications of bioturbation, due to the lack of such information from the southeastern Arabian Sea. 3.2. Westernmost IPWP (Core SK237 GC09) The core SK237 GC09 covers the last 184 kyr (Fig. 3). We did not find any correlation between Mg/Ca and Fe/Ca, Al/Ca and Mn/Ca, suggesting a proper cleaning of foraminiferal tests. The down-core variation in SST follows a clear glacial-interglacial trend, throughout the last ~184 kyr. Both the last (MIS5e) and current interglacials (Holocene) were warmer than the preceding glacial intervals. MIS5e SST was only marginally higher than the modern core site annual average SST, but 1.53 °C warmer than the core top SST. The lowest SST during the last two glacial maxima (24.28 °C and 24.73 °C, respectively) differed by ~0.5 °C. The average central equatorial Indian Ocean SST during MIS5 was 26.85 °C with the maximum (28.78 °C) at 127 kyr. The average westernmost IPWP (SK237 GC09) SST during the last glacial interval was 25.63 °C, similar to that during MIS6. The coolest SST (23.72 °C) was at 34.1 kyr. The last glacial interval was equally cool as MIS6. The coolest SST during the last glacial maximum (2.87 mmol/ mol; 24.28 °C) was at 22.1 kyr. Thus the LGM was 3.34 °C cooler than MIS5e. Although the early Holocene SST was the warmest in the current interglacial (10.2 kyr, 28.41 °C), no apparent trend is observed from ~9 kyr onwards till the core top. The core top is ~3 ka BP, and thus did not allow reconstructing modern SST from the area. The core top (0–1 cm) SST (27.25 °C) is 1.34 °C lower than the modern annual average SST (28.59 °C) at the core site. The annual average SST in the area is 28.59 °C (Locarnini et al., 2006) and is 3.35 °C higher than the LGM (Fig. 3). The most depleted δ18Oruber (−2.04‰) was during MIS5e. The average δ18Oruber during the last glacial interval (MIS2–4) was −0.33‰ (varying from −1.04 to 0.22‰), and was the same as that during MIS6. The δ18Oruber during LGM was −0.03‰. The average δ18Oruber during the present interglacial (MIS1) was −0.99‰ and was 0.21‰ enriched than the last interglacial. The δ18Osw was consistently higher than other published records from the IPWP region during all the glacial-interglacial cycles. The most enriched δ18Osw was at 14.2 kyr and the most depleted was at 53.6 kyr. The maximum δ18Osw was at 11.3 kyr (Fig. 3).

2.1.1. Isotopic and elemental analysis The elemental (Mg/Ca) and stable oxygen isotopic (δ18O) ratio of surface-dwelling planktic foraminifera Globigerinoides ruber (white) was analyzed to reconstruct SST and evaporation-precipitation changes. For Mg/Ca analysis, 40–50 clean specimens of G. ruber from 250 to 355 μm size range were picked, crushed under clean glass plates and transferred to centrifuge tubes. The specimens were cleaned following the UCSB standard foraminifera cleaning procedure without the DTPA step (Lea et al., 2000; Martin and Lea, 2002). Thoroughly cleaned and dissolved samples were analyzed by using a Thermo Finnigan Element2 sector field ICP-MS following the isotope dilution/internal standard method (Martin and Lea, 2002). To monitor the effectiveness of cleaning, concentration of Al, Fe and Mn was also measured, simultaneously. The SST was calculated from the Mg/Ca ratio by using a depth corrected calibration equation (Dekens et al., 2002).

Mg/Ca = 0.38 exp 0.09

[SST

0.61

(core depth in km)]

The standard error in SST calculated by using this equation was estimated to be ± 1.2 °C, when fit to Atlantic and Pacific core-tops. For δ18O analyses, 15–20 clean specimens of G. ruber from 250 to 355 μm size range were used. The stable oxygen isotopic ratio was measured at the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven) using a Finnigan MAT 253 isotope ratio gas mass spectrometer, coupled to an automated carbonate preparation device (Kiel IV) and calibrated via NBS 19 to the VPDB scale. The precision of δ18O was < 0.08‰, based on the repeat analysis of an in-house standard (Solnhofen limestone), measured with the samples over one year. Foraminiferal δ18O depends on both the ambient temperature and seawater oxygen isotopic ratio (δ18Osw). The δ18Osw was calculated by subtracting Mg/Ca derived SST contribution from δ18Oruber, by using Bemis et al. (1998) equation. 18O sw

= 0.27 + ((SST

16.5 + 4.8

4. Discussion

18O ruber )/4.8)

The cumulative error in δ18Osw, based on the error associated with the Mg/Ca and δ18O measurements is estimated at ± 0.3‰.

4.1. The long-term glacial-interglacial SST changes The difference in core top and modern SST is attributed to the older age of the sediments (3.1 kyr) and lack of modern sediments in the core. The difference may also be due to the lack of regional calibration equation for the Indian Ocean region. We have used the dissolution corrected calibration equation to estimate SST from G. ruber Mg/Ca (Dekens et al., 2002). The partial dissolution of foraminiferal tests may also bias the Mg/Ca SST (Fehrenbacher and Martin, 2014). The core site is, however, well above (1400 m) the carbonate lysocline (4400 m, Banakar et al., 1998). The calcium carbonate is also high throughout the core (40.3 ± 6.8%), confirming that the core has always been well above the carbonate lysocline. Additionally, we did not observe any apparent dissolution signature in the tests and carefully picked intact specimens. The core-top Mg/Ca being < 4.0 mmol/mol, at such a warm site, however, indicates some degree of dissolution. MIS5e SST was warmer as compared to preindustrial time. Similar

3. Results 3.1. Chronology The top section of the core was dated by four accelerator mass spectrometer radiocarbon dates on mixed planktic foraminifera, measured at the Center for Applied Isotope Studies, the University of Georgia, USA. The 14C dates were calibrated by using Calib7.0 software and MARINE13 dataset (Table 1) (Stuiver et al., 2018). The chronology of the older section was established by comparing the stable oxygen isotopic (δ18O) ratio of surface-dwelling planktic foraminifera Globigerinoides ruber (white) with the LR04 global isostack (Lisiecki and Raymo, 2005) (Fig. 2). A total of 14 tie points, representing major marine isotopic stage (MIS) boundaries were used. The core section 3

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Table 1 The details of Accelerator Mass Spectrometer radiocarbon dates, used to establish the chronology of the top section of the core. Sample Code

Depth Interval (cm)

14

C Age (yr BP)

14

C Age Error (±)

Calib. Age Range (1σ) (yr, BP)

Calib. Age-Range (2σ) (yr, BP)

Calib. Age (Median Probability) (yr, BP)

UGAMS#5382 UGAMS#5383 UGAMS#5384 UGAMS#5385

00–01 25–26 49–50 74–75

3693 12,935 27,664 37,060

41 100 413 688

2979–3180 12,267–1253 26,677–2701 35,707–36,000

2879–3273 12,098–12,583 26,534–27,134 35,572–36,125

3083 12,382 26,841 35,851

MIS5e warming was also reported in another record from the central equatorial Indian Ocean (Saraswat et al., 2005). The penultimate glacial maximum was warmer than the LGM as also observed in the central equatorial Indian Ocean (Saraswat et al., 2005; Tachikawa et al., 2008). Additionally, the lowest SST during the last glacial maximum, lasted for a comparatively longer duration, than the penultimate glacial maximum. The longer duration of the last glacial maximum as compared to the penultimate one may be due to the difference in sample resolution. The 3.35 °C drop in SST during LGM, as compared to the modern annual average SST at the core site, agrees well with other open ocean records from the tropical Indian Ocean. The northeastern Arabian Sea SST was also 3–4 °C colder during the LGM (Anand et al., 2008; Govil and Naidu, 2010). The drop in LGM SST is, however, higher (0.7 °C) than that in a core (SK237 GC04) collected from a relatively shallower depth (1245 m) in the southeastern Arabian Sea (Saraswat et al., 2013). In the central equatorial Indian Ocean also, LGM SST was 2.0–2.5 °C lower (Saraswat et al., 2005; Mahesh and Banakar, 2014), suggesting a relatively less cooling than that observed in the westernmost IPWP. SST difference during T1 and T2 (calculated as the difference between the lowest SST during the glacial maximum and the warmest SST during the interglacial) was comparable (4.13 °C and 4.01 °C, respectively) in the southeastern Arabian Sea. This SST difference during T1 and T2 in the southeastern Arabian Sea was 1.4–1.9 °C more than the SST difference in the central equatorial Indian Ocean (Saraswat et al., 2005). A comparable SST contrast during the last two glacial-interglacial transitions in both cores suggests a basin-wide consistent response to the warming forcing including insolation (Laskar et al., 2004) and atmospheric CO2

(Petit et al., 1999). The modern annual average core site SST is same for both the southeastern Arabian Sea (SK237 GC09) as well as that central equatorial Indian Ocean (SK157 GC04). A large drop in SST in the southeastern Arabian Sea during the last glacial maximum, thus suggests an increased SST gradient in this region. 4.2. The Indo-Pacific Warm Pool Structure 4.2.1. The Penultimate Glacial Period (MIS6) Several published SST and δ18O records (Table 2) were compiled to understand the temporal variation in the extent and SST gradient of the IPWP (Fig. 4). SST anomaly was calculated by subtracting the modern SST at the respective core site, from the Mg/Ca SST at each interval in the core (Fig. 5). For each core, the δ18Osw was estimated from temperature corrected δ18O (Fig. 5). Out of the six cores studied from the IPWP, only 4 cover MIS6 interval. The spatial coverage, however, is extensive enough to visualize the IPWP structure throughout its modern extent. The cores are from the central, western Pacific Ocean and the central Indian Ocean and cover both the eastern and western margins of the IPWP. The data were also compared with a core from the eastern equatorial Pacific Ocean (TR163–19), to understand the relationship between IPWP and El-Nino Southern Oscillation structure. The longterm glacial-interglacial SST trend in all cores from the IPWP region matches very well with the atmospheric CO2 concentration (Fig. 5), suggesting a close link between IPWP temperature and atmospheric CO2. The regional factors, including the insolation and transport of water from the Pacific into the Indian Ocean through the Indonesian Fig. 2. The chronology of core SK237 GC09. The blue stars mark four Accelerator Mass Spectrometer radiocarbon dates. The black filled diamonds are the tie points based on the comparison of the δ18OG. ruber with the global isostack LR04 (Lisiecki and Raymo, 2005). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4

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Fig. 3. The Mg/Ca (mmol/mol), δ18OG.

ruber

(‰) and δ18Osw (‰) data of SK237 GC09. The gray shaded bands represent interglacial Marine Isotopic Stages (MIS).

Table 2 The details of the cores from the IPWP region, as discussed in the text. Sr. No.

Core

Latitude (°)

Longitude (°E)

Depth (m)

Reference

1. 2. 3. 4. 5. 6.

SK237 GC09 SK157 GC04 GeoB 10,038–4 MD97–2140 ODP806B ODP871

12.01 2.40 −5.93 2.03 0.31 5.55

70.87 78.00 103.24 141.76 159.36 172.34

3001 3500 1891 2547 2520 1254

This study Saraswat et al., 2005 Mohtadi et al., 2010 de Garidel-Thoron et al., 2005 Lea et al., 2000 Dyez and Ravelo, 2013

throughflow, however, modulated sub-orbital changes in IPWP temperature. The entire equatorial region was cooler than the present, throughout the MIS6. The degree of cooling throughout the equatorial Indian and the western Pacific Ocean was the same from the beginning of MIS6, till ~160 kyr (Fig. 5). The SST anomaly in the eastern equatorial Pacific Ocean was also the same as that in the western Pacific and the Indian Ocean. The cooling in the central Pacific Ocean (ODP871) during the same interval was, however, relatively less, thus altering the zonal SST gradient. The warmer central equatorial Pacific Ocean (ODP871) during the glacial interval, suggests a considerably reduced IPWP extent. SST anomaly in the central equatorial Indian Ocean and the eastern equatorial Pacific Ocean matches very well throughout the MIS6, until the deglaciation. SST anomaly in the equatorial Indian Ocean and the equatorial western Pacific Ocean was different for a brief interval between ~145 - ~160 kyr BP. During this interval, the SST anomaly in the western Pacific Ocean was less than that in both the equatorial Indian and the eastern Pacific Ocean. The central Pacific Ocean (ODP871), was the warmest region of the equatorial Indian and the Pacific Ocean, throughout the MIS6. The western equatorial Pacific Ocean (ODP806B, MD97-2140) was cooler

than the central equatorial Pacific Ocean. The westernmost region of the IPWP (SK237 GC09) was also cooler and the difference in central equatorial Pacific Ocean and the Indian Ocean SST increased with the progress of MIS6. The warming of central equatorial Pacific Ocean as compared to the eastern equatorial Pacific Ocean suggests eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific Ocean. A comparatively higher average temperature (≥1.5 °C) in the central equatorial Pacific Ocean as compared to the eastern Pacific and the Indian Ocean, due to the eastward shift of the warm waters suggests a change in IPWP structure and also a weaker Walker circulation (Wara, 2005; Vecchi et al., 2006; Williams and Funk, 2011; Tokinaga et al., 2012; Bayr et al., 2014). A consistently warmer (> 28 °C) central equatorial Pacific and significantly cooler equatorial western Pacific and the Indian Ocean, suggests that the warm pool extent reduced considerably during the MIS6. A weaker Walker circulation implies warmer eastern Pacific Ocean. SST anomaly in the eastern Pacific Ocean, however, was the same as that in the western Pacific as well as the Indian Ocean. The comparable SST anomaly in the eastern and western Pacific as well as the central Indian Ocean, suggests the influence of factors other than Walker circulation in modulating the IPWP structure. The difference in SST, however, 5

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Fig. 4. The Mg/Ca (mmol/mol) (black) (A) and δ18OG. ruber (‰) (blue) (B) of the cores from the IPWP that cover the last glacial interglacial interval: SK237 GC09, this study; SK157 GC04, Saraswat et al., 2005; GeoB 10,038–4, Mohtadi et al., 2010; MD97–2140, de Garidel-Thoron et al., 2005; ODP806B, Lea et al., 2000; ODP871, Dyez and Ravelo, 2013. The δ18OG. ruber for core ODP871 is not available. The y-axis scale is same in all figures, for ease of comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

decreased towards the end of MIS6. Interestingly, the equatorial eastern Pacific Ocean warmed considerably towards the end of MIS6, so much so that the SST in the entire equatorial region was same, within the error limits. The warming during the end of MIS6 is attributed to a change in polar ice sheets. The increased solar insolation melted the ice sheets leading to upwelling in the Southern Ocean. The widespread Southern Ocean upwelling drove a CO2 increase in the atmosphere and the concomitant rise in global SST (Denton et al., 2010). The SST in the IPWP region responds to the solar insolation and atmospheric CO2 concentration. The SST in the Pacific Ocean is, however, more sensitive to greenhouse gas concentration (Lea, 2004; Dyez and Ravelo, 2013; Tachikawa et al., 2014). The SST gradient in the Pacific Ocean linearly responds to atmospheric CO2 (Yang et al., 2016). We demonstrate that the factors other than atmospheric CO2 affect SST in the equatorial Indian and the Pacific Ocean. The increase in SST difference between

the Indian and the Pacific Ocean with the progress of MIS6 is attributed to the relatively lower sea level (Grant et al., 2014). The lower sea-level reduced cross-basin exchange that serves as the conduit of water and heat flow into the Indian Ocean from the Pacific Ocean through the Indonesian through-flow region. The increase in sea-level towards the end of MIS6 resulted in a more vigorous transport of warm Pacific water into the Indian Ocean and thus comparable SST throughout the equatorial Indian and the Pacific Ocean. The equatorial Indian Ocean δ18Osw was enriched throughout the MIS 6 (ranging from 1.59–2.42‰) as compared to the equatorial western and eastern Pacific Ocean. The average δ18Osw in the equatorial western and eastern Pacific Ocean was 0.78‰ and 0.60‰, respectively, ~1.2‰ lower than the average in the equatorial Indian Ocean. The average global ice volume contribution during the studied interval of MIS6 was 0.8‰ (Waelbroeck et al., 2002). Subtracting the global ice volume induced enrichment, still leaves ~1.0‰ change in δ18Osw in the 6

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Fig. 5. The long-term SST (°C), SST anomaly and δ18Osw (‰) data from the IPWP (SK237 GC09, ODP806B, OPD871 and MD97–2140) and the eastern equatorial Pacific Ocean (TR163–19) plotted with the global mean sea level (m) (Grant et al., 2014), atmospheric CO2 (ppmv) (Petit et al., 1999) and June month's solar insolation (W/m2) at the equator (Laskar et al., 2004). The shaded bands represent alternate Marine Isotopic Stages.

equatorial Indian Ocean. A considerable enrichment of δ18Osw in the equatorial Indian Ocean, despite a comparable SST anomaly, clearly suggests that the δ18Osw in the equatorial Indian Ocean was strongly modulated by the weakened regional monsoon. A peculiar feature of the δ18Osw during the MIS6 is the opposite trend in the Indian and the Pacific Ocean between ~180 kyr and ~160 kyr. The equatorial Indian Ocean δ18Osw enriched by ~0.70‰ during this interval and the Pacific Ocean δ18Osw depleted by ~0.5‰. The change in δ18Osw during the later MIS6 was similar in both the basins, though the difference remained. The enriched δ18Osw throughout the penultimate glacial interval suggests weaker summer monsoon, the major source of precipitation in the Indian subcontinent and the adjacent oceanic region. A majority of the records suggest a weaker summer monsoon during the

glacial intervals (Saraswat et al., 2014). The large drop in δ18Osw despite a comparable change in SST, suggests that the monsoon forcing has been much stronger in altering the surface hydrography in the eastern Indian Ocean than the central Pacific Ocean. 4.2.2. The last interglacial (MIS5) A total of six records, including the present core, cover the entire MIS5 in the IPWP region. The extent of SST anomaly in the records increased considerably, as compared to MIS6. In the eastern Indian (GEOB10038–4) and the central Pacific Ocean (ODP871), however, the SST anomaly was within ± 1 °C, for a major part of the MIS5. The maximum negative SST anomaly (~2 °C) was in the westernmost region of the IPWP (SK237 GC09), for the major part of MIS5, implying an 7

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increase in meridional SST gradient during the last interglacial. The zonal SST gradient decreased considerably, throughout the core IPWP, during MIS5 as compared to MIS6. The westernmost margin of the IPWP, however, was cooler than the core IPWP. The equatorial Indian Ocean region south-east of the present core, as well as the entire western equatorial Pacific Ocean, was ~1 °C warmer. The SST in the westernmost IPWP was same as that in the eastern equatorial Pacific Ocean, during the MIS5e. The eastern equatorial Pacific Ocean, however, became cooler with the progress of the last glacial interval, thus altering the zonal SST gradient in the equatorial Pacific and the Indian Ocean. Incidentally, the eastern equatorial Pacific Ocean was significantly cooler than other parts of the IPWP. The difference in SST between the eastern equatorial Pacific Ocean and the rest of the IPWP was the maximum during 107–112 kyr as well as 83–90 kyr, suggesting weaker El-Nino. The east-west SST difference leads to stronger Walker circulation and a northward shift of ITCZ. The SST in the IPWP dropped by ~2 °C from MIS5e to MIS5a. The entire equatorial IPWP region, except the westernmost part, was equally warm with the only small difference in SST, throughout the MIS5. Interestingly, the SST in the westernmost margin of the IPWP (SK237 GC09) was also the same as the rest of the IPWP between ~88–98 kyr and 75–80 kyr. The westernmost part of the IPWP became warmer than the threshold 28 °C, during the MIS5e, suggesting the westward extension of the IPWP. The reduced zonal SST gradient in the core IPWP region during MIS5 indicates that the increased heat transfer from the western Pacific region influences the SST of central and eastern Indian Ocean during the interglacial interval. The reduced zonal SST gradient is attributed to the enhanced cross-basing heat transport facilitated by the high sea-stand during the last interglacial interval (Fig. 5). Based on the records from the eastern equatorial Indian Ocean, the sea level was suggested as the dominant factor controlling zonal SST gradient in the IPWP during the glacial-interglacial transition (Mohtadi et al., 2010). The cooler eastern Pacific Ocean SST is attributed to the warming of the Indian Ocean SST as higher SST in the Indian Ocean helps in the transition from El-Nino to La-Nina stage by generating upwelling Kelvin waves (Kug and Kang, 2006). The similar SST trend throughout the MIS6 and MIS5 further confirms a close link between the westernmost IPWP and the eastern equatorial Pacific Ocean. A majority of the records have a consistent δ18Osw trend, with the most enriched values during the MIS5e, followed by a large depletion (~1.50‰) between 121 and 130 kyr. Out of all the records, however, the largest variability during the MIS5 is observed in the eastern equatorial Indian Ocean (GeoB 10,038–4). Within the central equatorial Indian Ocean, despite the difference in SST in the core and marginal IPWP, the δ18Osw was same during the larger part of the MIS5 except for the MIS5a. The eastern equatorial Indian Ocean was, however, less saline than the central equatorial Indian Ocean, suggesting stronger precipitation in the eastern equatorial Indian Ocean, as compared to the central equatorial Indian Ocean. The western equatorial Pacific Ocean δ18Osw was consistently depleted than the Indian Ocean δ18Osw throughout the MIS5. The most depleted δ18Osw during the MIS5 was in the eastern equatorial Pacific Ocean. The decreased δ18Osw difference between the Pacific and Indian parts of the IPWP, during the last interglacial, once again confirms a pivotal influence of Indonesian Throughflow in modulating the hydrography of this region.

Bengal and the Arabian Sea modulates the SST in the southeastern Arabian Sea and central equatorial Indian Ocean. Any change in the intensity of seasonally reversing currents is likely to influence the SST in this region. The higher SST difference between SK157 GC04 and SK237 GC04 is attributed to the stronger winter monsoon current and weaker summer monsoon current during the last glacial interval. The stronger winter monsoon current enhanced the transport of cooler Bay of Bengal water, thus decreasing the SST at SK237 GC04 as compared to the equatorial core. The difference in SST at these two core sites decreases during the transition. The decreased difference is attributed to strengthened summer monsoon current as it brings warmer water to the southeastern Arabian Sea. The eastern equatorial Indian Ocean SST was cooler by up to ~1 °C, than the central equatorial Indian Ocean (SK157 GC04). The highest SST difference was during the last glacial-interglacial transition. The lower degree of cooling (1.2 °C) in the eastern equatorial Pacific Ocean (Koutavas et al., 2002), as compared to the western Pacific and the Indian Ocean (~3 °C) reduced the zonal SST gradient. The reduced zonal SST gradient ensued a weaker Walker circulation, El-Nino like conditions and southward shift of ITCZ (Koutavas et al., 2002; Dinezio and Tierney, 2013). A strengthening of the Indian Walker circulation was also suggested (Mohtadi et al., 2017) during the LGM. Out of all the records from the IPWP, the most enriched δ18Osw was in the central equatorial Indian Ocean and the most depleted δ18Osw was in the eastern equatorial Pacific Ocean. A large drop in δ18Osw (~0.7–1.0‰) is seen from ~65 kyr to ~55 kyr, in a majority of the records. Subtracting the sea-level contribution of ~0.3‰ during this interval (Waelbroeck et al., 2002), the remaining 0.4–0.7‰ change in δ18Osw suggests strengthened precipitation throughout the IPWP. From ~55 kyr onwards, a majority of the Indian Ocean records show an enriched δ18Osw trend. δ18Osw increased by ~1.0–1.5‰ until the LGM. The global seal level contribution during the corresponding interval was only ~0.6‰. The remaining, 0.4–0.9‰ increase in δ18Osw suggests progressively weakened precipitation, throughout the larger part of the last glacial interval. A large increase in δ18Osw in the equatorial Indian Ocean, without a simultaneous change in SST anomaly, further confirms the strong influence of local monsoon in modulating the hydrography of the equatorial Indian Ocean, especially during the glacial intervals. The weakened glacial precipitation has been reported in several records from the Indian Ocean (Chodankar et al., 2005; Govil and Naidu, 2010; Mahesh et al., 2011). The eastern equatorial Indian Ocean δ18Osw was same as that in the central equatorial Indian Ocean, suggesting negligible hydrographic gradient in the eastern and central equatorial Indian Ocean. The eastern equatorial Indian Ocean δ18Osw was, however, depleted, as compared to the southwestern Arabian Sea (SK237 GC09). The enriched eastern equatorial Indian Ocean δ18Osw as compared to the southwestern Arabian Sea, suggests weaker precipitation due to a southward shift of ITCZ during the glacial interval. Interestingly, the western equatorial Pacific Ocean δ18Osw was the same as the eastern equatorial Pacific Ocean, throughout the MIS2–4, suggesting negligible salinity gradient in the equatorial Pacific Ocean. 4.2.4. Present interglacial (MIS1) A majority of the records have a uniform SST anomaly throughout the Holocene. The extent of SST anomaly during the Holocene was the same as that during the last interglacial (MIS5). We do not, however, observe a decreasing SST anomaly trend during the Holocene as evident during MIS5. The eastern equatorial Pacific Ocean was the coldest, as compared to the IPWP, throughout the MIS1. The difference in SST between the eastern equatorial Pacific and the IPWP was the maximum during the early Holocene and decreased subsequently, with the warming of the eastern equatorial Pacific Ocean. The average SST at a few sites (SK237 GC09, GeoB10038–4) within the modern IPWP, was lower than the threshold (28 °C), until the mid-Holocene. The SST at the rest of the sites within the core IPWP was consistently warmer than 28 °C, throughout the MIS1. The SST at the westernmost IPWP site

4.2.3. The last glacial interval (MIS 2-4) Before the transition, SST was uniform throughout the MIS2-4 in a majority of the records. Interestingly, in several records (SK157 GC04, ODP871, ODP806B, GeoB10038-4), the SST during the last glacial maximum (LGM) (19–24 kyr) was not significantly cooler than during the rest of the MIS2-4. The central equatorial Indian Ocean (SK157 GC04) SST was, however, warmer than the region further north (SK237 GC09). The difference in SST at these two sites of the central Indian Ocean varied throughout the last glacial period, being the maximum during the transition. The seawater exchange between the Bay of 8

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(SK237 GC09) was also consistent, with an average of 27.46 °C, throughout the present interglacial. The mid-Holocene cold interval had La-Nina like condition with enhanced atmospheric circulation and higher SST gradient (Koutavas et al., 2002) and weakening of ENSO at 6.5 kyr (Tudhope et al., 2014). The La-Nina like condition existed during early Holocene (4–11 kyr), and the subsequent demise of northern hemisphere insolation and southward movement of ITCZ resulted in El-Nino like conditions in recent times (Gagan and Thompson, 2004). The δ18Osw continuously depleted from the early to middle Holocene, suggesting increased precipitation during this interval. Although the extent of depleted δ18Osw varies from the core to core, the strengthened monsoon pattern is consistent throughout the IPWP. The δ18Osw was, however, uniform from ~6 kyr, until the present, at a majority of the sites, suggesting stabilization of the monsoon. The most enriched δ18Osw (~1‰ higher than the rest) was in the westernmost IPWP (SK237 GC09). Here, also, the δ18Osw depleted by ~1‰, from the early Holocene to mid-Holocene, suggesting weakened monsoon. The most depleted δ18Osw, was in the eastern Indian Ocean, south of the equator (SO189-39KL).

Warm Pool structure as well as its relationship with other regional phenomena, during the last ~184 kyr BP. The entire equatorial region was cooler than present during MIS6. The degree of cooling throughout the equatorial Indian and the eastern, as well as the western Pacific Ocean, was similar from the beginning of MIS6 until ~160 kyr. The cooling in the central Pacific Ocean during the same interval was, however, relatively less, thus altering the zonal SST gradient and suggesting a strong La-Nina like condition. The most enriched δ18Osw during MIS6 was in the westernmost IPWP, suggesting weakened precipitation, despite the La-Lina like condition. The extent of SST anomaly in the records increased considerably during MIS5 as compared to MIS6 and thus the zonal SST gradient decreased considerably, throughout the core of the IPWP. The westernmost IPWP, however, was cooler than the core of the IPWP, suggesting a decrease in the spatial extent of the IPWP. The eastern equatorial Pacific Ocean warmed considerably during the MIS5e, suggesting a weaker Walker Circulation and a strong El-Nino. The difference in δ18Osw decreased considerably during MIS5e and is attributed to increased precipitation/freshwater runoff in the central equatorial Indian Ocean. The interval from ~65 kyr to ~55 kyr is marked by a large drop in δ18Osw (~1‰) in a majority of the records, suggesting strengthened precipitation throughout the IPWP. From ~55 kyr onwards, a majority of the Indian Ocean records show an enriched δ18Osw trend. The increase in δ18Osw is ~1.5‰, suggesting progressively weakened precipitation throughout the larger part of the last glacial interval. Interestingly, in several records (SK157 GC04, ODP871, ODP806B, GeoB10038–4), the SST during the last glacial maximum (LGM) (19–24 kyr) was not significantly cooler than during the rest of the MIS2–4. A majority of the records have a uniform SST anomaly throughout the Holocene. The δ18Osw continuously became more depleted from the early to middle Holocene, suggesting increased precipitation during this interval. Although the extent of depleted δ18Osw varies from the core to core, the strengthened monsoon pattern is consistent throughout the IPWP. The δ18Osw was, however, uniform from ~6 kyr until the present, at a majority of the sites, suggesting stabilization of the monsoon. The shift from the glacial maximum to the warmest interglacial temperature was longer during the penultimate transition.

4.2.5. Transitions A similar SST pattern is observed during both the last two transitions. A decrease in SST anomaly, suggesting the beginning of the last deglacial warming, is observed at ~18 kyr in a majority of the cores and continued throughout the transition. Interestingly, SST anomaly was negligible in the central equatorial Pacific Ocean throughout the transition, including LGM. A comparatively smaller SST anomaly is also observed in the central equatorial Indian Ocean (SK157 GC04). Contrary to that, the maximum SST anomaly was in the westernmost IPWP (SK237 GC09). The SST anomaly in the eastern equatorial Pacific Ocean was also the same as that in the eastern equatorial Indian Ocean. The different SST anomaly at these sites, suggests a strong influence of regional factors, in addition to the change in solar insolation and atmospheric CO2. The central equatorial Indian Ocean was warmer than the westernmost IPWP (SK237 GC09) during the last transition, same as that throughout the large part of the last glacial interval. The difference in SST between the central equatorial Indian Ocean and the westernmost IPWP region, was as high as 4 °C, at ~18 kyr. The SST difference between these regions decreased towards the end of the transition. The increased SST difference in these regions is attributed to the enhanced transport of Bay of Bengal water into the southwestern Arabian Sea (Mahesh et al., 2011; Saraswat et al., 2013). The higher warming (~1 °C) of the equatorial central Pacific Ocean than the western Pacific Ocean, also resembles conditions same as that during the MIS6, suggesting weaker Walker circulation. A semi-permanent La-Nina like condition prevailed during Termination-I enforcing the initial warming in the IPWP region (Koutavas et al., 2002; Stott et al., 2002). A large difference is observed between the westernmost IPWP δ18Osw and the rest of the equatorial region. The westernmost IPWP δ18Osw was enriched by ~1‰ as compared to a majority of the records from the IPWP, suggesting a weaker monsoon throughout the transition. The high-resolution centennial records also have a similar δ18Osw pattern in both the eastern (SO189-39KL) and the central (SK237 GC04) Indian Ocean, suggesting a similar forcing controlling the hydrography in these regions, during the deglaciation. A similar late deglaciation drop in δ18Osw is, however, not evident in the westernmost IPWP (SK237 GC09).

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